Thin film transistors (TFTs) are a type of field effect transistors (hereinafter referred to as FETs). TFTs are three-terminal elements having a gate terminal, a source terminal, and a drain terminal in the basic structure. TFTs are active elements having a function of switching the current between the source terminal and the drain terminal so that a semiconductor thin film deposited on a substrate is used as a channel layer in which electrons or holes move and a voltage is applied to the gate terminal to control the current flowing in the channel layer. TFTs are electronic devices that are most widely used these days in practical application. Typical applications of TFTs include liquid-crystal driving elements.
Currently, most widely used TFTs are metal-insulator-semiconductor-FETs (MIS-FETs) in which a polycrystalline silicon film or an amorphous silicon film is used as a channel layer material. MIS-FETs including silicon are opaque to visible light and thus fail to form transparent circuits. Therefore, when MIS-FETs are used as switching elements for driving liquid crystals in liquid crystal displays, the aperture ratio of a display pixel in the devices is small.
Due to the recent need for high-resolution liquid crystals, switching elements for driving liquid crystals now require high-speed driving. In order to achieve high-speed driving, a semiconductor thin film in which the mobility of carriers, electrons or holes, is higher than that in at least amorphous silicon needs to be used as a channel layer.
Under such circumstances, Patent Document 1 proposes a transparent semi-insulating amorphous oxide thin film which is a transparent amorphous oxide thin film deposited by vapor-phase film deposition method and containing elements of In, Ga, Zn, and O. The composition of the oxide is InGaO3(ZnO)m (m is a natural number less than 6) when the oxide is crystallized. The transparent semi-insulating amorphous oxide thin film is a semi-insulating thin film having a carrier mobility (also referred to as carrier electron mobility) of more than 1 cm2/(V·sec) and a carrier density (also referred to as carrier electron density) of 1016/cm3 or less without doping with an impurity ion. Patent Document 1 also proposes a thin film transistor in which the transparent semi-insulating amorphous oxide thin film is used as a channel layer.
However, as proposed in Patent Document 1, the transparent amorphous oxide thin film (a-IGZO film) containing elements of In, Ga, Zn, and O and deposited by any method of vapor-phase film deposition selected from sputtering and pulsed laser deposition has an electron carrier mobility in the range of only about from 1 to 10 cm2/(V·sec). It is pointed out that this carrier mobility is insufficient to further improve the definition of displays.
Patent Document 2 discloses a sputtering target for the purpose of forming the amorphous oxide thin film described in Patent Document 1, that is, a sintered body target containing at least In, Zn, and Ga. The sputtering target contains In, Zn, and Ga and has a relative density of 75% or more and a resistance ρ of 50 Ωcm or less. However, since the target disclosed in Patent Document 2 is a polycrystalline oxide sintered body having a homologous-phase crystal structure, an amorphous oxide thin film obtained by using the target has a carrier mobility of only about 10 cm2/V·s as well as in Patent Document 1.
Regarding materials for achieving high carrier mobility, Patent Document 3 proposes a thin film transistor including an oxide thin film in which gallium is dissolved in indium oxide. In the oxide thin film, the Ga/(Ga+In) atomic ratio is 0.001 to 0.12, and the percentage of indium and gallium with respect to the total metal atoms is 80 at % or more. The oxide thin film has an In2O3 bixbyite structure. An oxide sintered body is proposed as the material of the oxide thin film in which gallium is dissolved in indium oxide. In the oxide sintered body, the Ga/(Ga+In) atomic ratio is 0.001 to 0.12, and the percentage of indium and gallium with respect to the total metal atoms is 80 at % or more. The oxide sintered body has an In2O3 bixbyite structure.
However, when a crystalline oxide semiconductor thin film as proposed in Patent Document 3 is applied to TFTs, a problem associated with variations in TFT characteristics due to crystal grain boundaries arises. In particular, it is significantly difficult to uniformly form a TFT on an eighth or later generation large glass substrate.
Patent Document 4 describes an oxide sintered body having a bixbyite structure and containing indium oxide, gallium oxide, and zinc oxide, and having a composition range that satisfies the formula of In/(In+Ga+Zn)<0.75 in terms of the atomic percentage of indium (In), gallium (Ga), and zinc (Zn). In TFT evaluation, Examples in which the mobility is as high as about 20 cm2/V·s are disclosed.
However, there is problem in that microcrystals or the like tend to be generated in an oxide semiconductor thin film formed from the sintered body disclosed in Patent Document 4. In particular, it is difficult to form a TFT on a large glass substrate with a high yield. A typical process for fabricating an oxide semiconductor thin film transistor involves once forming an amorphous film and then forming an amorphous or crystalline oxide semiconductor thin film by annealing. After the amorphous film formation process, wet etching with a weak acid, such as an aqueous solution containing oxalic acid or hydrochloric acid, is performed in order to pattern the amorphous film into a channel layer having a desired shape. However, when the oxide sintered body substantially composed of a bixbyite structure disclosed in Patent Document 4 is used, the formed amorphous film has a low crystallization temperature, and there is a problem in that microcrystals are already formed at a stage after film deposition and residues are generated in an etching process, or in that partial crystallization hinders etching. That is, it is difficult to form a TFT channel layer with a desired pattern by wet etching using a photolithography technique or the like, or if a TFT is formed, problems associated with unstable operation or the like arise.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2010-219538    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2007-073312    Patent Document 3: PCT International Publication No. WO2010/032422    Patent Document 4: PCT International Publication No. WO2009/148154
Non-Patent Document 1:    A. Takagi, K. Nomura, H. Ohta, H. Yanagi, T. Kamiya, M. Hirano, and H. Hosono, Thin Solid Films 486,38 (2005)